Before you send a message, Microsoft Outlook automatically checks the names you type in the To, Cc, and Bcc boxes against the names in the Address Book and against those contact folders that you've specified as Outlook Address Books. If an exact match is found, the name is underlined. If multiple matches are found, a red, wavy line appears under the name. Right-click the name to see the matches.
If multiple matches are found, and you have used the address before, the name you chose previously is displayed in the box with a green, dashed underline to remind you that there are other matches. Right-click the name to see the matches.
You can also manually check names in messages by clicking the Check Names button on the message toolbar